Have to lower the thermastate to 40 for it to cut on

I would replace it exactly the same way you removed it.

usually if screws into the lower part of the radiator.

Copy following link for replacing gas control valve thermostat on water heater:
http://waterheatertimer.org/How-to-replace-thermostat-on-gas-water-heater.html
Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Is your check engine light on? If so, scan for trouble codes. Also, check for vacuum leaks, distributor cap & rotor.

it might be your thermastate stuck close because it lets the collant to go into the engine chambers so if your thermastate is stuck is why it is over heating so you can take the thermastate out and test it yourself by putting it into water inside a pan that is on a stove to test to see if it opens up for you but also there are different thermastates out there that have different temperature ranges that you can put in place of that one so go to autozone they will explain to you which one could help you out the most.

I assume you are talking about an electric water heater. If so, then it is possible for the thermostats to be stuck, but a nore common cause for realy hot water on these is the lower element going out due to hard water. When the hard water deposits build up over the lower element and thermostat it will sometimes continue to heat because the thermostat does not get an accurate reading. It will usualy do this untill the lower element blows. If you have very hard water or the unit is very old, you may want to replace the lower element. Be sure to get a good replacement. Not a cheap one.

Check that you have no air locks or blockages .If you have a fan assisted engine .it should cut in at a certain temp

Use a fine tooth, some what like a miter saw would have. works well.

A hanger may have come loose from the deck that needs to be reattached.
Plus check your tire pressure, one tire with less air than the other will make it cut lop sided.

thermistate housing is the place you refer to it fuction is to hold a thermastate to open and close water flow at 150 to 190 degree according to car spectif the thermaste sticks it causes overheating and can blow the thermastate gasget, the tole cost is about 30$ at most for the thermastate and gasget...now many other things can cause a car to over heat and cause a hose leak and somtimes it is simple to just tighten the hiose clamp at the thrmastate and problem sovled see a water leak causes the radiator to lose presure and thus with out a sealed tight water cooling system the boiling point of water is lower and thus the car over heats when in doubt stop by the local gas station and have a tech take a quick look as it could be simple as the hose clamp difficult as a therm,astate but do not let it go to warp a head gasket as this blown headgasket is what is the end results of running a car to hot